The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures due to oxidation and/or hot corrosion when located in certain high-temperature sections of a gas turbine engine, such as the turbine, combustor or augmentor. Examples of such components include buckets (blades) and nozzles (vanes) in the turbine section of a gas turbine engine. A common solution is to protect the surfaces of such components with an environmental coating system, such as an aluminide coating, an overlay coating or a thermal barrier coating system (TBC). The latter includes a layer of thermal-insulating ceramic adhered to the superalloy substrate with an environmentally-resistant bond coat.
Metal oxides, such as zirconia (ZrO.sub.2) that is partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or another oxide, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer is typically deposited by air plasma spray (APS), vacuum plasma spray (VPS), also called low pressure plasma spray (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure. APS is often preferred over other deposition processes because of low equipment cost and ease of application and masking. Notably, the adhesion mechanism for plasma-sprayed ceramic layers is by mechanical interlocking with a bond coat having a relatively rough surface, preferably about 350 microinches to about 750 microinches (about 9 to about 19 .mu.m) Ra.
Bond coats are typically formed from an oxidation-resistant alloy such as MCrAlY where M is iron, cobalt and/or nickel, or from a diffusion aluminide or platinum aluminide that forms an oxidation-resistant intermetallic, or a combination of both. Bond coats formed from such compositions protect the underlying superalloy substrate by forming an oxidation barrier for the underlying superalloy substrate. In particular, the aluminum content of these bond coat materials provides for the slow growth of a dense adherent aluminum oxide layer (alumina scale) at elevated temperatures. This oxide scale protects the bond coat from oxidation and enhances bonding between the ceramic layer and bond coat.
Aside from those formed by diffusion techniques and physical or chemical vapor deposition, bond coats are typically applied by thermal spraying, e.g., APS, VPS and high velocity oxy-fuel (HVOF) techniques, all of which entail deposition of the bond coat from a metal powder. The structure and physical properties of such bond coats are highly dependent on the process and equipment by which they are deposited. The surface preparation requirements for a substrate on which a VPS bond coat is to be applied are typically different from that required for APS and HVOF bond coats. Relatively small grit sizes (typically about 60 to about 120 .mu.m) are used to grit blast a substrate before applying a VPS bond coat, which usually results in a substrate surface roughness of less than about 200 microinches Ra (about 5 .mu.m). Vacuum heat treatment is typically applied after VPS to diffusion bond the bond coat to the substrate.
In contrast, grit sizes of about 170 to about 840 .mu.m are typically used to grit blast substrates on which an APS or HVOF bond coat is to be applied. Because the adhesion mechanism between a substrate and an APS and HVOF bond coat is by mechanical interlocking, these bond coats do not typically undergo a vacuum heat treatment prior to deposition of the thermal barrier coating. Air plasma possesses a high heat capacity in the presence of air, which enables relatively large particles to be melted using APS. As a result, coarser metal powders can be used that yield bond coats having a rougher surface, e.g., in the 350 to 750 microinch range suitable for adhering a plasma-sprayed ceramic layer, than is possible with VPS. The particle size distribution of such powders is Gaussian as a result of the sieving process, and are typically broad in order to provide finer particles that fill the interstices between larger particles to reduce porosity. However, the finer particles are prone to oxidation during the spraying process, resulting in a bond coat having a very high oxide content. The low momentum possessed by the sprayed particles in the APS process also promotes porosity in the coating. Consequently, as-sprayed APS bond coats inherently contain relatively high levels of oxides and are more porous than are VPS bond coats. Because of their higher level of oxides and porosity, APS bond coats are more prone to oxidation than are VPS bond coats.
As indicated above, HVOF bond coats do not undergo a vacuum heat treatment before deposition of a thermal barrier coating, since adhesion of an HVOF bond coat to its substrate is by mechanical interlocking. Bond coats deposited by HVOF techniques are very sensitive to particle size distribution of the powder because of the relatively low spray temperature of the HVOF process. Accordingly, HVOF process parameters have been typically adjusted to spray powders having a very narrow range of particle size distribution. To produce an HVOF bond coat suitable for a plasma-sprayed ceramic layer, a coarse powder must typically be used in order to achieve the required surface roughness. However, because coarse particles cannot typically be fully melted at suitable HVOF parameters, HVOF bond coats of the prior art have typically had relatively high porosity and poor bonding between sprayed particles.
In view of the above, it can be seen that, while bond coats deposited by various techniques have been successfully employed, each has advantages and disadvantages that must be considered for a given application. In particular, while APS processes readily yield a bond coat having adequate surface roughness to adhere a plasma-sprayed ceramic layer, porosity and the tendency for oxidation in such bond coats are drawbacks to the protection and adhesion they provide to the underlying substrate. Because of poor bonding between particles, oxygen readily diffuses into HVOF bond coats subjected to a high-temperature oxidation environment, causing oxidation of the bond coat at the multiple surfaces of the loosely bonded particles.
Accordingly, what is needed is a process by which the surface roughness necessary for a plasma-sprayed ceramic layer can be achieved with a bond coat that also exhibits low porosity and oxidation.